The present invention relates to an electro-photographic type double-sided image forming apparatus such as a copying machine, printer and FAX, in which a toner image formed on an image forming body is transferred onto both sides of the recording sheet for fixing.
Heretofore, in a double-sided copy, an image on one side formed on an image forming body is transferred onto a recording sheet for fixing. The aforesaid recording sheet is temporarily housed in a double-sided reversal paper feeding device. The aforesaid recording sheet is re-fed from the double-sided reversal paper feeding device synchronously with an image formed on the image forming body for transferring an image on the other side onto the recording sheet for fixing.
In the aforesaid double-sided copying device, as described above, conveyance of the recording sheet is conducted such as feeding of a paper to the double-sided reversal paper feeding device and double passage to the fixing device. Therefore, reliability on conveyance of the recording sheet is reduced, causing jamming. As described above, the above-mentioned image forming apparatus in which images are formed on both sides using the double-sided reversal paper feeding device requires much time for copying since the conveyance distance of the recording sheet is lengthened. In addition, since the recording sheet receives heat twice for fixing, paper quality is damaged. To the contrary, technologies to fix images with one passage after forming toner images on both sides of the recording sheet are disclosed in Japanese Tokkousho Nos. 37538/1974 and 28740/1979 and Japanese Tokkaihei 1-44457 and 4-214576.
In the above-mentioned double-sided image forming apparatus, toner images provided on both sides of recording sheet are simultaneously fixed due to heating by the fixing device once only. Therefore, there is no possibility to hurt paper quality of the recording sheet and copying speed can be increased by shortening the conveyance path of the recording sheet.
However, in this apparatus, the toner image formed by an image forming means is transferred to an intermediate transfer body. A next image is formed after one rotation of the intermediate transfer body. Therefore the image forming means needs waiting time to form the next image, and this waiting time remained as the cause decreasing the copying speed.
The present inventors further continued reduction of copying speed. An objective of the present invention is to realize an image forming apparatus capable of extremely increasing copying speed. Further, another objective of the present invention is to provide an image forming apparatus capable of forming images on one side additionally to the aforesaid functions and to provide an image forming apparatus capable of conducting treatment when the image forming unit does not serve for image forming.